Apparatus for production of spherical grain ferrite powder

ABSTRACT

Apparatus for producing substantially spherical grain ferrite powder comprising a vertical cylindrical vessel in which a slurry of ferrite-forming powdered raw materials in alcohol is sprayed under applied pressure through a spray nozzle into a high temperature atmosphere of between 700°-1500° C. to allow the combustion of sprayed alcohol therein, thereby instantaneously producing spherical grain-ferrite powder. A hot air guide is provided along the inner surface of the wall of the vertical vessel extending upward in a spiral for providing a hot air current rising spirally upwardly within the vessel to provide a high temperature atmosphere above the spray nozzle.

This is a division, of application Ser. No. 823,345, filed Aug. 10, 1977now abandoned.

The present invention relates to an apparatus for the production of aspherical-grain ferrite powder. More particularly, the present inventionrelates to an apparatus for the instantaneous formation of aspherical-grain ferrite powder in a continuous manner.

Heretofore, a ferrite powder such as one to be used for a carrier in thedry process copying machine has generally been produced by a processcomprising a number of separate steps such as, for example, a step forgranulation and drying of raw material which is usually carried out in aspray drier and a separate step for sintering the drier product which isusually carried out in a muffle furnace, a rotary kiln or the like.

This type of prior art process, however, includes a number of drawbacks.For example, it generally requires two or three separate steps. Inaddition, if the sintering is carried out in a muffle furnace, aresidence time of at least 2 hours and, sometimes, even as long as 10hours is required. Alternatively, if the sintering is carried out in arotary kiln, troublesome problems such as adhesion of clinkers to theinner surface of the kiln or agglomeration of grains often occur. Thus,the process for the production of homogeneous and spherical-grainferrite powder has been accompanied by a number of difficulties.

Accordingly, the object of the present invention is to provide a novelapparatus for the production of a spherical-grain ferrite powder whichcan ensure the instantaneous production of the above-mentioned ferritepowder directly from ferrite-forming raw materials or and ferrite-formedraw materials.

According to their experiments, the present inventors have found that bymaking a suspension in alcohol of various ferrite-formed andferrite-forming raw materials and spraying said suspension into a hightemperature zone, all the steps required for the production of ferritesuch as the steps of granulation, ferrite-formation for such forming rawmaterials and sintering can be accomplished instantaneously andspherical-grain ferrite powder is readily obtained. Thus, the process ofthe present invention comprises (1) the step of dispersing finelypowdered ferrite-forming raw materials in alcohol, (2) the step ofspraying the resulting suspension into a hot air zone at a temperaturein the range of 700° to 1500° C. and (3) the step of keeping the sprayedraw materials in the same zone to carry out the granulation,ferrite-formation and sintering of the same while it remains in saidhigh temperature zone.

The term "ferrite-forming, finely powdered raw material or materials"herein used include a wide variety of different materials. Through thetracking experiments, the present inventors have confirmed that almostall of those mixtures which can form a compound having the genericformula: MO.Fe₂ O₃, wherein M represents a divalent metal, are usable inthe practice of the present invention. Representative examples of suchmaterials can be classified in the manner as shown below.

(a) Oxide-type or carbonate-type materials:

The combinations of (1) Fe₂ O₃ and one or two or more of these types ofmaterials included Mn, Ni, Zn, Co, Mg, Cu, Li, and the like as well asthe combinations of (1) Fe₂ O₃ and (2) one or two or more of Ba, Sr andthe like.

(b) The simple- or complex-spinel or hexagonal types of ferrite powdersprepared by the solid phase reactions from the materials given in (a) ata temperature in the range of 800°-1500° C.

(c) Fine grains, singly or in mixture, of simple-spinel type ferritehaving an average grain size of 50-100 A prepared by the "neutralizationmethod" according to the following reactions:

    M.sup.2.spsp.+ +2 Fe.sup.3.spsp.+ +ROH→MO.Fe.sub.2 O.sub.3

wherein M represents Mn, Ni, Zn, Co, Mg and the like, and R representsNa, K, NH₄ and the like.

(d) Simple- or complex-spinel type ferrite grains having an averagegrain size of 0.1-1 micron prepared by the "oxidation method" accordingto the following reactions.

    M.sup.2.spsp.+ +2 Fe.sup.3.spsp.+ +ROH+O.sub.2 ↑→MO.Fe.sub.2 O.sub.3

wherein M represents Mn, Co, Mg, Zn and the like and R represents Na, K,NH₄ and the like.

(e) Hydroxide or carbonate or a mixture of both to be used as a rawmaterial for hexagonal ferrite each containing one or two or more ofBa².spsp.+, Sr².spsp.+, and the like with Fe³.spsp.+ prepared by the"co-precipitation method".

These ferrite-formed or ferrite-forming raw material powders are thensuspended in an alcohol. A number of various alcohols can be used forthe purpose. For example, methyl alcohol for industrial use which iseasily available can conveniently be used in the practice of the presentinvention. The preferred range of solids content of the slurry isexperimentally determined for each case depending on the raw materialsspecifically used, the operating conditions specifically employed andthe like, but a solids content of 10-70% by weight is generallyemployed.

The operations of spraying this slurry into a high temperature zone andof keeping the sprayed materials in the same zone can be carried out ina continuous manner if an apparatus having the structure as shown inFIG. 1 in the attached drawing is employed, and the production of highlyhomogeneous quality and spherical-grain ferrite powder is ensured. Theabove mentioned operations will be further explained below referring tothe apparatus shown in the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a vertical section of one preferred example of theapparatus of the present invention which can be used conveniently in thepractice of the process of the present invention.

FIG. 2 shows a horizontal section of the apparatus shown in FIG. 1 atthe position of A--A' plane.

FIG. 3 shows a longitudinal section of one preferred example of theslurry spraying nozzle usable in the process of the present invention.

FIG. 4 shows a scanning type electron microscope photograph of thespherical-grain ferrite powder obtained by the process of the presentinvention.

DETAILED DESCRIPTION

In the drawing, the numerical symbols represent the following things,respectively. 1: refractory brick; 2: insulating brick; 3: slurry spraynozzle; 4: hot air inlet; 5: waste gas outlet; 6: hot air guiding means;7: hole for measuring temperature (also used as a peep hole); 8:reaction product outlet.

Referring to FIGS. 1-3, the apparatus of the present invention comprisesa vertical cylindrical vessel having a lining of refractories(refractory brick 1 and insulating brick 2). A jet nozzle 3 from whichthe slurry is to be sprayed is provided in the center of the bottom. Atthe lower portion of the wall is provided a hot air inlet 4 and anexhaust outlet 5 is provided at the top. A hot air guiding means 6 forexample in the form of an open groove extends upwards in the shape of aspiral from said hot air inlet 4 along the inner surface of the wall. Ahigh temperature zone is formed in the vessel by a stream of hot airrising along said guiding means 6 provided on the inner surface of thewall. At the same time, a slurry of raw materials in alcohol iscontinuously sprayed through the nozzle 3 in the direction of the axialflow of the rising air current, namely in the direction of the axis ofthe cylinder. In FIG. 1 the numerical symbol 7 represents atemperature-checking hole (which is also a peep hole), 8 represents anoutlet for the reaction products, 9 represents an iron-made shell, 10represents heat insulating material and 11 represents a strut pole.

The ratio of diameter to height (R/L) of the apparatus is about 1/4.

In the practice of the present invention, hot air at a temperature ofabout 1200° C. provided by a kerosene burner is introduced into theabove mentioned apparatus from the inlet 4 and at the same time a slurryof raw materials suspended in alcohol is sprayed vertically upwards fromthe jet nozzle 3. In this manner, alcohol burns and the steps ofgranulation, ferrite-formation and sintering of the powdered rawmaterials are accomplished instantaneously.

Sintered grains thus produced fall in the reaction vessel and areremoved from the product outlet 8. The product thus obtained isimmediately available for various uses as a spherical-grain ferritepowder.

In the process of the present invention, the combustion of alcohol inthis way seems to enable the instantaneous accomplishment ofgranulation, ferrite-formation and sintering of the raw materialscomprising fine grains on the order of microns and helps to standardizethe ferrite quality and form spherical grain shapes. Accordingly, theoperation is normally carried out in an atmosphere having the remainingO₂ concentration in the region of 3-5%. In some cases, however,depending on the raw materials employed, better results can be obtainedby operating the process either in the more oxidizing or in the morereducing atmosphere than that mentioned above.

As regards the temperatures of the high temperature atmosphere, when theabove mentioned type of ferrite-forming raw materials are employed, goodresults can be obtained for temperatures ranging from 700° and 1500° C.As a nozzle 3 for spraying the slurry, simple type nozzle having thestructure as shown in FIG. 3 can be used conveniently.

The body of this nozzle consists of a heat resisting metal and thenozzle-tip portion thereof or its portion of contact with liquid of theslurry is made of a wear resisting and corrosion resisting materialwhich consists primarily of ceramics.

The raw material slurry is pressure-fed to the nozzle by a plunger pumpor from the pressure tank at a pressure for example in the region of2-50 kg/cm².

The grain size of the desired spherical-grain ferrite can be controlledby the proper selection of various factors including the diameter of thenozzle, the pressure applied to the slurry, the solids concentration ofthe slurry and the like. The following examples will further illustratethe present invention.

EXAMPLE 1

Finely powdered oxides of Ni, Zn and Fe each having an average grainsize of 1.0 micron or less were weighed respectively so as to obtain theformulation of 15% by weight of NiO, 35% by weight of ZnO and 50% byweight of Fe₂ O₃ and were suspended in methyl alcohol for industrial useto provide a slurry having a solids content of 45%. The resulting slurrywas transported to a jet nozzle under applied pressure of 2 kg/cm² andwas spurted from the nozzle into the reaction vessel having thestructure as shown in FIG. 1 in the attached drawing, the inside ofwhich was held at a temperature in the range of 1200°-1250° C. by thecombustion gas from a kerosine burner introduced therein from the hotgas inlet. A series of reactions including granulation,ferrite-formation and sintering were carried out instantaneously, andthe powdered product was obtained. This powder was confirmed by X-raydiffraction analysis to have the Ni-Zn type ferrite composition. Thispowder had the saturation magnetization (σ_(s)) of 50 emu/g and adensity (d) of 5.0 g/cm³. Grains were spherical and 90% thereof were inthe range of 30-200 microns in their grain size. The scanning typeelectron microscope photograph of the product grains was as shown inFIG. 4.

EXAMPLE 2

Finely powdered carbonates and oxides of Cu, Zn and Fe used as rawmaterials each having an average grain diameter of 1.0 micron or lesswere weighed respectively so as to obtain the formulation of 25% byweight of CuO, 25% by weight of ZnO and 50% by weight of Fe₂ O₃ and theywere suspended in methyl alcohol for industrial use to provide a slurryhaving a solids content of 40%. Except that this slurry was used as rawmaterial for spraying, the same procedures as in Example 1 wererepeated. The powdered product thus obtained was Cu-Zn ferrite havingthe saturation magnetization (σ_(s)) of 38 emu/g and a density of 4.9g/cm³. Grains were spherical and 90% thereof were in the range of 30-200microns in their grain size.

EXAMPLE 3

Finely powdered oxides and carbonates of Mg, Zn and Fe each having anaverage grain size of 1.0 micron or less were weighed respectively so asto obtain the formulation of 25% by weight of MgO, 25% by weight of ZnOand 50% by weight of Fe₂ O₃ and then they were suspended in methylalcohol for industrial use to provide a slurry having a solids contentof 50%. Except that this slurry was used as a raw material to be sprayedinto the reaction vessel, the same treatment as in Example 1 wasrepeated. Mg-Zn ferrite having the saturation magnetization (σ_(s)) of40 emu/g and a density (d) of 4.7 g/cm³ was obtained. Grains having thegrain size of 50-250 microns made up 80% of the total.

EXAMPLE 4

Carbonates and oxides of Ba and Fe each having an average grain size of1.0 micron or less were weighed respectively so as to obtain theformulation for BaO.5.5Fe₂ O₃ and they were suspended in methyl alcoholfor industrial use to provide a slurry having a solids content of 50%.This slurry was transported and sprayed under applied pressure of 2.5kg/cm² into the reaction vessel as shown in FIG. 1 in the attacheddrawing and kept at a temperature in the range of 1200°-1300° C. Thespherical-grain barium ferrite powder thus produced in the apparatusafter finishing granulation, ferrite-formation and sintering wasobserved to possess the saturation magnetization (σ_(s)) of 58 emu/g.This powder also comprises highly spherical grains as in previousexamples and had a density (d) of 4.9 g/cm³ and 90% of the grains werein the range of 30-200 microns in their grain size.

EXAMPLE 5

The same procedures as in Example 4 were repeated except that SrO wasused instead of BaO. The produced spherical-grain strontium ferrite hada magnetic saturation value (σ_(s)) of 53 emu/g, a density (d) of 4.7g/cm³, and 90% of the grains were in the range of 30-200 microns intheir grain size.

EXAMPLE 6

NiFe₂ O₄ and ZnFe₂ O₄ both prepared by the "neutralization method" weredried at low temperatures. Then, using these compounds as a raw materialpowder, both were weighed respectively to obtain a composition havingthe same formulation as in Example 1. The above composition was treatedunder the same conditions as in Example 1 and a nickle-zinc ferrite wasprepared. The product thus obtained was a spherical-grain powder and itscharacteristic properties included the saturation magnetization (σ_(s))of 55 emu/g, and a density (d) of 4.9 g/cm³. The grain size thereof wassuch that 90% of the powder was in the range of 50-200 microns.

EXAMPLE 7

A mixture of hydroxides and carbonates of Ba and Fe containing 12 Feions per 1 Ba ion was precipitated from an aqueous solution by the"co-precipitation method", and the precipitate was fully washed anddried. The fine-grain powder thus obtained was then dispersed in methylalcohol for industrial use to provide a slurry having a solids contentof 40%. Then, the slurry was transported and spurted under appliedpressure of 3 kg/cm² into the inside of the reaction vessel shown inFIG. 1 when the temperature therein reached 1200°-1250° C.

The powdered product obtained after a series of reactions carried out inthe vessel including granulation, ferrite-formation and sintering wasbarium-ferrite having the saturation magnetization (σ_(s)) of 49 emu/g,and a density (d) of 4.9 g/cm³, and the grain size thereof was such that90% of the total grains were in the range of 50-200 microns.

As is obvious from the above examples, a soft and a hard ferrite can beobtained instantaneously from a variety of different ferrite-forming rawmaterials according to the process and apparatus of the presentinventions. Moreover, the resulting ferrite is homogeneous in qualityand comprises highly spherical grains and is of extremely high quality.

Heretofore, it has been considered that one of the best ways forimproving the characteristic properties of ferrite material is to carryout the pressing or compacting of mixed raw materials before suchmixture is subjected to sintering. Considering this fact, the process ofthe present invention has really brought about countless advantages: thesimplification of operation steps, shortening of reaction time and thesensitive controlability of the production process due to the increasednumber of controllable operation factors.

What we claim is:
 1. Apparatus for the production of substantiallyspherical grain-ferrite powder which comprises:a vertical cylindricalvessel having a lining of refractory material; a hot air intake providedat the lower portion of the wall of said vertical vessel; an exhaustvent provided at the top of said vertical vessel; a spray nozzleprovided in the center of the bottom of said vertical vessel andoriented to provide a continuous jetting feed of a ferrite powdersuspension rising in the same direction as the axis of said verticalcylindrical vessel; an open groove hot air guide provided along theinner surface of the wall of said vertical vessel, said open grooveopening to the interior of said vertical vessel and extending upward ina spiral form starting from said hot air intake and toward said exhaustvent for providing a hot air current rising spirally upwardly withinsaid vessel and around said sprayed suspension to produce a hightemperature atmosphere above said spray nozzle for the formation ofsubstantially spherical-grain ferrite particles from said sprayedsuspension; and an outlet at the lower end of said vessel for removal ofsaid substantially spherical-grain ferrite particles.
 2. Apparatusaccording to claim 1, wherein said hot air guide produces a hightemperature atmosphere in the range of from about 700° to about 1500° C.3. Apparatus according to claim 1, wherein said nozzle jets in asuspension of ferrite powder in alcohol.